...What? For one, dividing the exp reward among the number of available party members is precisely how exp is supposed to work in D&D, generally speaking.
That aside, precisely based on this principle solo runners would exp WAY faster than large parties (but of course the downside would be facing few uphill battles to get there, since they don't have support from others). And D&D has never been about strict level requirements to begin with. It's more about approximate ranges, which is something it can afford thanks to the progression curve not being particularly steep (if not for some occasional ramp ups in power when certain milestones are reached).
Also, I'm not sure why we are talking about it in terms of "Can you imagine what a mess that would be? " when that's precisely how plenty of other games did it in the past (once again, including BG1 and BG2). And yeah, of course once you get to the absolute, final level cap allowed by the game the solo player would potentially be at disadvantage over a full party of the same level... But that's precisely what solo players usually strive for: the additional challenge. Being able to say "I did it, despise the odds being stacked against me".